1. Field of the Invention
The present invention relates to a process for producing a hydrogen-containing gas by means of steam-reforming reaction of methanol. More particularly, the present invention pertains to a process for producing a hydrogen-containing gas which comprises subjecting methanol to steam-reforming reaction through a auto thermal reaction in the presence of oxygen.
2. Description of the Related Arts
It has hitherto been well known that methanol is reformed into a hydrogen-containing gas in the presence of a catalyst in a comparatively easy manner. In particular, methanol, which is reformed by reaction with steam into a hydrogen-containing gas having a low concentration of carbon monoxide which is difficult to separate, has recently become a center of attraction as a simple convenient supply source of hydrogen which is expected to expand its demand in the near future.
In addition, since a fuel cell, especially that which is employed for automobiles is called upon to be miniatured and simplified in its structure, development is set forward on a reactor of a auto thermal reaction in which reaction heat necessary for steam-reforming reaction is supplied by the heat of combustion through the introduction of air into the reactor.
The above-mentioned steam-reforming reaction of methanol in which methanol is reformed into a hydrogen-containing gas by the use of steam is constituted of the principal reaction as represented by the chemical equation (1) and besides the reverse shift reaction by producing a small amount of carbon monoxide as represented by the chemical equation (2).CH3OH+H2O=3H2+CO2+49.5 kJ/mol  (1)CO2+H2=CO+H2O+41.17 kJ/mol  (2)
The amount of the carbon monoxide which is by produced in the chemical equation (2) is preferably as small as possible, but the carbon monoxide is difficult to remove in the case of purifying into highly pure hydrogen. From the aspect of thermodynamic equilibrium, the concentration of carbon monoxide in the reformed gas can be lowered with lowering in the reaction temperature and with an increase in the molar ratio of steam to methanol (hereinafter referred to as “S/C ratio”).
The principal reaction of the steam-reforming reaction of methanol as represented by the chemical equation (1) is an endothermic reaction, to which heat must be supplied from the outside of the reaction system, and accordingly suffers from such disadvantages that heat supply equipment is made necessary and thus the production unit becomes intricate and troublesome. There is available against an auto thermal reaction in which air is introduced in the reaction system along with methanol and steam so as to oxidize part of methanol, and the resultant reaction heat is utilized to cause the steam-reforming reaction of methanol as represented by the chemical equation (1) to take place. In the above-mentioned process, part of methanol is oxidized into hydrogen and carbon dioxide as represented by the chemical equation (3) so that the resultant reaction heat is utilized to proceed with the steam-reforming reaction of methanol as represented by the chemical equation (1).CH3OH+½O2=2H2+CO2−192.3 kJ/mol  (3)
The aforesaid process is characterized in that heat supply is needed until the reaction temperature reaches a level necessary for commencing the reaction, but is not needed when the reaction is once continued.
A number of proposals have been made on a catalyst to be employed in the steam-reforming reaction of methanol, including the catalyst on which are supported a base metal element such as copper, nickel, chromium, zinc and aluminum and the oxide or the like thereof, and the catalyst on which a platinum series metal such as platinum and palladium is supported on a carrier such as alumina. For instance, there are proposed the catalyst comprising a platinum series metal as an active ingredient in Japanese Patent Application Laid-Open No. 1742371/1983 (Showa 58) and the like, and the catalyst comprising nickel as a principal ingredient in Japanese Patent Application Laid-Open No. 49204/1975 (Showa 50) and the like.
However, in the presence of a catalyst comprising such a metal other than copper as a platinum series metal and nickel, a methanol decomposing reaction predominantly proceeds according to the chemical equation: CH3OH→CO+2H2, thereby bringing about the drawback in that the above-mentioned catalyst is ineffective from the aspect of producing hydrogen.
That is to say, the catalyst effectively proceeding with the reaction: CH3OH+H2O→CO2+3H2 which is advantageous for effective production of hydrogen from methanol has heretofore been restricted to a catalyst comprising copper as an principal ingredient. As a catalyst comprising copper as a fundamental ingredient, there are proposed the catalyst which comprises copper, zinc and aluminum and the like catalyst {refer to Japanese Patent Publication No. 177/1995 (Heisei 7)}.
Nevertheless, the catalyst comprising copper as a fundamental ingredient suffers from such shortcomings as poor heat resistance at a high temperature of about 250° C. and higher and generation of sintering for copper and/or zinc as catalyst components during a long time service, thus causing deterioration in catalytic activity in a short period of time.
On the contrary, there are proposed the palladium-zinc oxide catalyst {Japanese Patent Application Laid-Open No. 49930/1993 (Heisei 5)}, platinum-zinc oxide catalyst {Japanese Patent Application Laid-Open No. 25662/2001 (Heisei 13)} and the like catalysts.
However, mention is made of high selectivity to steam-reforming reaction, that is, low selectivity to carbon monoxide in the reformed gas, but nothing is mentioned about sufficient heat resistance and durability on the above-mentioned Japanese Patent Application Laid-Open No. 49930/1993 (Heisei 5). The use of the platinum-zinc oxide catalyst {Japanese Patent Application Laid-Open No. 25662/2001 (Heisei 13)} brings about selectivity to carbon monoxide higher than that when use is made of the palladium-zinc oxide catalyst and besides, nothing is mentioned about sufficient heat resistance and durability thereon. Moreover, the aforesaid platinum-zinc oxide catalyst needs to be subjected to a reduction treatment with hydrogen at 200° C. outside of the furnace in the reaction system, and thus it is problematical under the aspect of maintenance when mounted on a portable methanol reforming apparatus such as on an automobile.
Further, the catalysts disclosed therein is concerned with the steam-reforming reaction of methanol without referring to the auto thermal reaction reaction.
In a reactor of the auto thermal reaction, part of methanol is oxidized, so that the portion where oxidative reaction takes place is brought to a high temperature as compared with the steam-reforming reaction, and thereby a catalyst having further high heat resistance is required.
Depending upon the type of a fuel cell, for instance, in the case of a polymer electrolyte fuel cell, an electrode catalyst is generally deteriorated by carbon monoxide in a definite concentration or higher and therefore, it is necessary to lower the carbon monoxide concentration to the extent that the electrode catalyst is not affected thereby through any possible means prior to the introduction of fuel gas in the fuel cell. In many cases carbon monoxide concentration is lowered to an order of ppm through the selective oxidative reaction, whereas limitations are put on a mounting capacity in vehicle mounting application. Thus in order to miniaturize a selective oxidative reaction unit for carbon monoxide, the amount of carbon monoxide to be treated is preferably as small as possible and at the same time, the concentration of carbon monoxide in a hydrogen-containing gas coming from a reforming unit is preferably as low as possible. In the case of a high carbon monoxide concentration, it is made necessary to separately equip an apparatus for lowering the concentration by means of water-gas shift reaction or the like, thus bringing about disadvantages in vehicle mounting application wherein limitations are put on a mounting capacity. Further in such application, a catalyst having high activity is called upon in order to miniaturize a reforming reactor according to the limitations on the mounting capacity.
Under such circumstances, the object of the present invention is to provide a process for producing a gas comprising hydrogen as the principal ingredient by means of a auto thermal reaction reactor in an industrially advantageous manner by developing a methanol reforming catalyst having a high activity, high heat resistance and high selectivity to steam-reforming reaction.
Other objects of the present invention will become obvious from the text of this specification hereinafter disclosed.